Apparatus and method for controlling molten metal pouring from a holding vessel

ABSTRACT

A molten metal holding and pouring apparatus ( 10 ) including a holding vessel ( 12 ) with a sealable chamber ( 22 ) and a pour spout ( 26 ) extending from the chamber ( 22 ), a gas supply assembly ( 40, 42, 44 ) formed and coupled to pressurize the chamber ( 22 ), and a control assembly controlling the pressure-induced outflow of molten metal ( 14 ) from the chamber ( 22 ) through the pour spout ( 26 ). The control assembly includes a pressure sensor ( 54 ) coupled to sense chamber pressure, a distance sensor ( 50 ) formed and positioned to directly sense the level of molten metal ( 14 ) in the chamber ( 22 ) without contacting the molten metal and a controller ( 80 ) responsive to the sensors ( 50, 54 ) to control operation of the gas supply assembly. Additionally, a pour spout level sensor ( 46 ) senses the level of the metal ( 14 ) in the pour spout ( 26 ) and signals a pour timer ( 76 ) or integration module ( 77 ) when each pour begins. The apparatus also includes a low flow rate capacity inlet valve ( 45 ) and pressure booster assembly ( 90, 96 ). A method of pouring molten metal ( 14 ) from the apparatus ( 10 ) is also disclosed.

TECHNICAL FIELD

The present invention relates, in general, to a novel and usefulapparatus and method for controlling the outflow of molten metal from aholding vessel or furnace, and more particularly, to an apparatus andmethod for controlling pressure-based pouring of molten metal from aholding vessel.

BACKGROUND ART

In the past, molten metals have been delivered to molds and castingmachines from furnaces by two general methods. One method employs amechanical apparatus in which pouring from the furnace or molten metalholding vessel is accomplished by tilting the furnace or vessel ordislodging a plug in the bottom of the furnace or vessel. Suchapparatus, although straightforward, are inaccurate and not easilycontrolled. They often result in spillage, the production of unusableparts, and injury to personnel. In addition, the molds or othercontainers which are employed to accept the molten metal are typicallyunder filled or overfilled. Moreover, mechanisms employed to effect suchmechanical emptying of metal holding vessels are expensive tomanufacture and operate. Because of such problems, the mechanicaltilting mechanisms for emptying molten metal furnaces or holding vesselshave largely fallen into disuse, while stopper plug devices remainexpensive to operate and maintain.

Another method for pouring molten metals from a furnace or holdingvessel uses pressurized gases. This method offers several advantagesover the older mechanical pouring mechanisms. Namely, pressure pouringapparatus are safer to use since a loss of pressure during such processresults in the molten metal remaining in the furnace and thereby doesnot tend to endanger personnel. Although useful in a batch deliverymode, pressure pouring furnaces or vessels still encounter problems withrespect to pouring accuracy. Moreover, it is important when successivepours of molten metal take place that each pour include a specificquantity of metallic material poured in the same specific interval oftime. In the past, pressure pouring systems have suffered in this regarddue to the fact that, as the molten metal level in the vessel decreases,there is a corresponding lowering of the metal level in the pouringspout or tube from which the metallic material egresses. Withoutcompensation for this effect, serial pours of declining quantity overincreasing time intervals would result. In the past, attempts to solvethis problem have suffered or failed due to the fact that detectors,compensation devices and controls are adversely influenced and evendestroyed by the molten metals, and are otherwise unable to accommodatethe changing internal geometry and dynamics of the process.

Accurately increasing the gas pressure in the molten metal holdingvessel during pouring operations has been recognized as necessary tomaintain precise pouring through the spout or pour outlet. In the past,such pressurizing has been accompanied by the use of submergedmechanical valves or the placement of an open orifice in the vessel ofsmaller size than the pouring spout. Such devices, althoughtheoretically functional, often cease to operate due to the malfunctionof mechanical parts and to erosion or clogging due to slag, sludge,dross, or other debris commonly found in molten metals.

Serial pouring of molten metal into molds has become extremely importantin industry as a result of the development of high speed sand castingmolding machines, fast indexing permanent mold turntables, and rapidcycling high pressure die casting machines. For example, high productionmolding lines are now capable of producing over 500 uncored sand moldsper hour. Filling these molds at a commensurate rate requires serialpours from a furnace that are rapid, very accurate, and highlyrepeatable. Current pressure-based molten metal pouring apparatus havenot been able to meet these demanding production goals, and pouring fromthe holding vessel has become a limiting factor in maintaining the highproduction rates necessary to achieve the desired economic benefits insuch molding lines.

The patent literature includes numerous “teapot” pressure pouring metalholding assemblies, often configured in the characteristic “teapot”design For example, U.S. Pat. No. 3,058,180 discloses a teapot holdingvessel system in which a float is employed at the outlet spout of theholding vessel. The float detects the level of the molten metal in thespout of the vessel and generates a signal which increases or decreasesthe pressure of the gas in the vessel to maintain a certain level ofmetal in the spout. The float, however, is easily fowled by slag,sludge, dross, and other common metal impurities, as well as beingeroded or destroyed by the heat of the molten metal.

U.S. Pat. No. 3,998,365 discloses a molten metal dispensing system forserial pouring of metal in which the gas pressure in the molten metalcontainer is successively increased by a predetermined amount with eachpour of metal. A contact probe is lowered into the pour spout to justbelow a level at which pouring will occur and the pressure in the sealedvessel raised until the molten metal reaches the probe. The probe isthen retracted and an incremental pressure is applied to the vessel fora timed interval to produce pouring of a desired quantity of metal. Thistype of pour spout level sensor is also known as an electrical “contactand withdraw” level sensor. At the end of the pouring time pressure isvented to lower the metal in the pouring spout, and thereafter the cycleis repeated. While this has some appeal as a solution, controlling thepressure of a compressible fluid is generally inaccurate even whenattempts are made to compensate for lowering levels of metal in thepressure vessel. The gas is a compressible spring whose propertieschange, and the much greater mass of the molten metal can cause surgingthat greatly complicates achieving accurate pours by trying to usepressure-based control systems.

In U.S. Pat. No. 4,220,319 a molten metal holding vessel is provided inwhich an electrical contact and withdraw sensor is used at the dischargespout to determine when molten metal has reached the pour threshold. Thepressure at which the pour threshold is reached is then maintained in adifferential pressure sensor assembly and a pressure increment appliedto the vessel and sensed. Such differential sensing allows a pressureadjustment in the vessel for lowering metal levels while maintaining thelevel of the molten metal at the discharge spout. This also is anexample of a pressure pouring system which employs an open loop pressurecontrol in which the pressurized gas is exhausted from the vesselchamber to atmosphere between successive pours, thus wasting compressedgas and increasing the time required to complete each pouring cycle.Such systems are more costly and less efficient and productive tooperate. They are also unable to automatically compensate orself-correct for normal system variables, such as gas leakage from thevessel chamber. Additionally, they attempt to control based upon gaspressure sensing, with the inherent loss of accuracy thatcompressibility produces.

The inability to control pours using the apparatus of U.S. Pat. No.4,220,319 was commercially addressed by adding a load cell to theassembly. The load cell measured the weight of the containment vesseland molten metal, and weight changes as a result of each pour were usedto determine changes in the level of molten metal in the vessel. Thislevel determination was used to attempt to improve the setting of thedifferential pressure used to make the pour. Nevertheless, pour accuracywas not as high as desired, particularly when the vessel was beingrefilled, and weight-based level sensing has its disadvantages,particularly with low density metals, as described below.

In German Patent No. DE 40 29 386 C2, a pressure pour furnace isdisclosed which includes a chamber pressure sensor and a molten metallevel sensor in the outlet or pour spout. An integration module also isprovided that integrates the chamber pressure over time, with theintegration starting when the metal level reaches the level sensor inthe pour spout. The pressure integration is used to determine pourvolume, but again the accuracy of the pours was not as high as would bedesirable because the system was controlling based upon pressuresensing.

My U.S. Pat. No. 3,499,580 employs a submerged ceramic bubbler tube tosense the pressure at a fixed level below the surface of the moltenmetal in a vessel. Continuous emission of gas bubbles enables adetermination of the height of the metal in the holding vessel pouringtube. Such measurement is then employed to control the pressure of thegas in the vessel chamber, forcing the metal from the vessel in order tomaintain a constant pour rate. Although effective, the bubbler tube issusceptible to erosion, clogging, and blockage if gas is not supplied tothe bubbler on a continuous basis.

U.S. Pat. Nos. 3,412,899, 4,445,670, and 4,730,755 recognize the need toincrease the gas pressure as pouring continues from a vessel on a batchbasis. Control systems in these references are based on load cellweighing systems which measure the quantity of molten metal in thevessel for each pour.

In U.S. Pat. No. 3,412,899 the sensed weight using the load cell iscombined with the sensed chamber pressure, but the load cell weightsensing is only an indirect and relatively insensitive method ofattempting to determine the level of the molten metal charge. Pressurepouring furnaces which employ a load cell to determine the metal levelin the holding chamber are inherently less sensitive for dispensing lowdensity metals, such as aluminum. This is because load cell accuracy isalways a percentage of the total amount weighed, and the load cell mustsense the combined weight of the holding vessel with all attachmentsincluding any heating apparatus as well as the molten metal charge. Thuswith low density metals, the weight of the molten metal charge, which isa smaller portion of the total weight sensed by the load cell, cannot bedetermined with sufficient accuracy to precisely infer the level ofmolten metal in the vessel. As a result, the accuracy of individualpours of low density metals is usually not commercially acceptable.Additionally, if the volume of the holding chamber of the vessel is notuniform, or if the heating apparatus affects the volume over the heightof the chamber, the load cell output will be non-linear and must becompensated for if the level of the charge in the holding chamber is tobe inferred from the load cell output.

In U.S. Pat. No. 3,412,899, therefore, the relatively insensitiveattempt to determine the location of the top surface of the metal in thechamber is combined with the pressure measurement to attempt to inferwhen the top surface of the metal in the pour spout reaches level N, thepour threshold. The system of the 3,412,899 patent is “controlled” bysensing the metal level in the mold and shutting down the pour, ratherthan any attempt to control the pour volume using the furnacecontroller.

In addition, prior art control systems including that of U.S. Pat. No.3,412,899 have not corrected for the increase in time required to fillthe gas volume in the sealed holding vessel above the molten metal asthe level of such molten decreases due to pouring. Thus, such prior artsystems are limited by the maximum capacity of the gas inlet valve for agiven fixed source pressure, and they continuously lose cycling speedand their pouring rates steadily diminish as the number of poursincreases in serial pour applications, which makes these systemscommercially unacceptable for today's high production requirements.

Yet another practice has been attempted in this field, namely, to ignorethe changing level of molten metal in a vessel due to pouring, and,instead, to have the system operator manually attempt to maintain aconstant level of molten metal in the holding vessel by frequentlyadding fresh molten metal to the vessel or furnace during the pouringoperation. However, such a refilling process still results in inaccuratemetal pours, and it is also inconvenient and labor intensive.

Accordingly, it is an object of the present invention to provide apressurized gas based, molten metal pouring apparatus, and method forcontrolling the operation of the same, which meters the outflow ofmolten metal from the holding vessel rapidly, accurately, and in arepeatable amount of time, regardless of the level of molten metal inthe holding vessel.

Another object of the present invention is to provide a molten metalpouring apparatus and method which is well suited for use in highproduction, serial pour, metal molding lines.

Another object of the present invention is to provide an apparatus andmethod for controlling the outflow of molten metal from a holding vesselor furnace which reduces costly spillage and reduces the under-fillingand over-filling of molds, casting machines, and other containersreceiving the outflow from the vessel.

A further object of the present invention is to provide an apparatus andmethod for controlling the outflow of molten metal from a holding vesselor furnace which is not susceptible to clogging due to impurities foundin the molten metal.

Another object of the present invention is to provide an apparatus andmethod for controlling the outflow of molten metal from a holding vesselor furnace which is safer to operate.

Another object of the present invention is to provide a molten metalpouring apparatus and method which is usable with many different kindsof molten metals including relatively hot and dense molten metals, suchas copper based alloys, iron and steel, as well as relatively lowerdensity metals, such as aluminum, zinc and magnesium.

Another object of the present invention is to provide an apparatus andmethod for accurately controlling the outflow of molten metal from aholding vessel which minimizes dynamic control system errors includingmetal surging and dynamic gas pressure oscillations.

A further object of the present invention is to provide an apparatus andmethod for accurately controlling the outflow of molten metal from aholding vessel which utilizes no moving or active parts or sensors indirect contact with the molten metal.

Still another object of the present invention is to provide apressure-based molten metal pouring apparatus and method which isrelatively inexpensive to construct and operate, overcomes gas leakage,requires minimal maintenance and repair, and is adaptable to a varietyof applications.

The molten metal pouring apparatus and control method of the presentinvention have other objects, features and advantages which will becomeapparent from, or are set forth in more detail in the following BestMode of Carrying Out the Invention and the accompanying drawing.

DESCRIPTION OF THE INVENTION

The molten metal pouring apparatus of the present invention comprises,briefly, a molten metal holding vessel having a sealable chamber with apour passageway extending from the chamber to a pour spout outlet; a gassupply assembly formed and coupled to supply gas to the chamber topressurize the chamber over the molten metal in order to control thelevel of molten metal in the pour passageway; and a pour controlassembly including a pressure sensor positioned to sense gas pressure inthe chamber, a chamber distance sensor formed and positioned to sensethe distance to a top surface of the molten metal in the chamber, a pourdistance sensor formed and positioned to sense the distance to a topsurface of the molten metal in the pour passageway, and a controllercoupled to the pressure sensor, the chamber distance sensor and the pourdistance sensor to receive signals therefrom, and the controller beingformed to be responsive to the signals from the sensors to control thesupply of gas from the gas supply assembly to the chamber.

The pressure sensor generates a signal representative of the gaspressure detected in the chamber. The controller advantageously includesa summing junction which adds the values of the pressure sensor signaland the chamber distance sensor molten metal level signal to produce aprocess variable signal, which is representative of the level of moltenmetal in the passageway. A set point module generates a signalindicative of a desired level of molten metal in the pour passageway.Such desired level may be to the point of pouring or at a point ready topour. The controller compares the process variable signal from thesumming junction with the set point signal from the set point module andgenerates a corresponding output signal.

The gas supply assembly includes a valve assembly and pneumatic circuitthat regulates the flow of pressurized gas from the source ofpressurized gas to the vessel chamber, as well as from the vesselchamber to atmosphere, preferably through a single conduit. The valveassembly is selected to include a relatively low flow rate capacityinlet valve so that controlling the inflow of gas to the chamber can beaccomplished with minimal pressure oscillation and the valve assembly isresponsive to the output signal from the controller to effect regulationof the flow of pressurized gas to and from the vessel.

In the present invention, compensation for the low flow rate capacity ofthe inlet valve is provided by a gas pressure booster pneumatic circuitand control mechanism. The booster circuit increases the rate of flow ofgas from a source of pressurized gas through the low flow rate capacityinlet valve to the vessel chamber as the level of molten metal in thevessel chamber diminishes. Such booster apparatus overcomes the maximumflow rate limitation imposed by the turndown ratio of the low flowcapacity inlet valve as higher flow rates are required to maintain theproduction pace. A computation control module, which receives the moltenmetal level signal from the chamber distance sensor, is preset togenerate an inverse output signal which passes to the pneumatic boostercircuit and increases gas flow through the inlet valve as the chambermetal level diminishes. A converter may be used to transduce anelectrical computation module signal to a pneumatic signal, if desired.

In a further aspect of the present invention, a pour spout distancesensor is provided which is formed and positioned to sense the level ofthe top surface of the molten metal in the pour passageway. The distancelevel sensor preferably senses the top molten metal surface withoutcontacting the same, and most preferable is a wave-operated sensor, suchas a radar wave-based sensor which is less sensitive to smoke, dust,steam, variable air temperatures and the like over the surface of themetal. The pour passageway distance sensor also may be a contact andwithdraw distance sensor, particularly if lower temperature moltenmetals are being poured.

If a pour passageway distance sensor is employed, a load cell basedinferential determination of the level of the charge in the holdingchamber can be employed, particularly if the holding chamber has arelatively uniform geometry over its height. Pour passageway distancesensing can be used to overcome small load cell level determinationinaccuracies.

The method of controlling the pouring of molten metal from a holdingvessel of the present invention is comprised, briefly, of the steps ofsensing the distance to the top surface of a charge of molten metal in asealed chamber of the holding vessel, sensing the pressure in thechamber over the molten metal, and pressurizing the chamber over themolten metal in response to the combination of the sensed distance andthe sensed pressure. Additionally, it is preferable that the presentcontrol method includes the step of sensing the distance to the topsurface of the molten metal in the pour passageway from the chamber.Both distance sensing steps are preferably accomplished using a radardistance sensing device that can sense the level of the metal withoutcontacting the same.

In another aspect of the method of the present invention a controlmethod is provided in which the flow rate of compressed gas to thesealed chamber is controlled by a low flow rate capacity inlet valvewhich can have its flow rate capacity increased or boosted as the metallevel in the chamber diminishes.

In a final aspect of the present invention, sensing of molten metallevels in one or both of the holding vessel chamber and the pourpassageway is accomplished using a distance sensor not requiring contactwith the molten metal, and most preferably level sensing is accomplishedusing a radar based distance sensor.

DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic, side elevation view, in cross section, of amolten metal pouring apparatus constructed in accordance with thepresent invention.

FIG. 2 is an enlarged, fragmentary view, in cross section, of the pourspout portion of the holding vessel apparatus depicted in FIG. 1.

FIG. 3 is a graphical representation of levels in the pour spout of theholding vessel at specific times during a pour cycle.

FIG. 4 is a block diagram corresponding to FIG. 1 depictingschematically the function of the control system of the presentinvention.

For a better understanding of the invention reference is made to thefollowing detailed description of the preferred embodiments thereofwhich should be referenced to the prior described drawings.

BEST MODE OF CARRYING OUT THE INVENTION

Turning now to FIG. 1 a molten metal holding and pouring apparatus,generally designated 0.10, is shown in which a molten metal holdingvessel 12 is depicted as a conventional pressure tight holding vessel.

Vessel 12 is properly insulated to hold molten metal charge 14therewithin, and includes a pressure tight top 16, bottom 18 and side 20defining a molten metal holding chamber 22. Vessel 12 can be a furnaceand most typically will have a heating assembly, not shown forsimplicity of illustration. Vessel 12 also includes a charging inlet 24which accepts a metal charge 14. Pour passageway or spout 26 extendsfrom chamber 22 to a pour spout outlet 28 to provide a path for pouringof metal charge 14 from chamber 22 of vessel 12. Mouth 28 of pourpassageway 26 lies a certain distance above a floor 30 of bottom 18 ofthe holding vessel. Mouth 28 preferably takes the form of a transverselyextending weir or orifice through which molten metal can flow at avolumetric rate that can be precisely determined.

Vessel or furnace 12 is of “teapot” configuration which is well known inthe art. In most embodiments of the invention, the vessel 12 will situpon a platform 32 which is supported by a fixed or moveable structure(not shown) attached to the ground surface 34, which is designed toaccommodate the attendant molding machine or casting process. In certainother embodiments of the invention, which will be discussed hereinafter,vessel 12 may sit upon a platform 32 which is itself supported aboveground surface 34 by fulcrum support member 36 and a load cell 38(depicted in broken lines). Although a single load cell 38 is depictedin FIG. 1, multiple load cells may be employed with apparatus 10 of thepresent invention. Moreover, as also will be explained, load cell 38 isnot a required nor preferred element of the broadest aspect of thepresent invention.

The molten metal holding vessel of the present invention employspressurized gas to effect pouring of metal from pour spout 26. It willbe seen, therefore, that a gas supply assembly is provided including apneumatic circuit or conduit array. Conduit 40 of that array can be seento extend from a pressurized source 42, here illustrated as apressurized gas container. Source 42 may be provided by dry compressedair, nitrogen, argon, or any gas which is compatible with molten metalcharge 14 within chamber 22 of vessel 12, that is, a gas that will notoxidize or otherwise react with and degrade the charge. The gas supplyassembly also includes a valve assembly, generally designated 44, whichregulates the flow of compressed gas from source 42 through conduit 40and into chamber 22 of holding vessel 12. The top surface 39 of moltenmetal 14 in the vessel chamber 22 lowers as gas pressure in chamber 22forces molten metal 14 out of the chamber 22 through pour passageway 26and out the pour spout outlet 43.

Valve assembly 44 of the present invention is preferably selected toinclude an inlet valve 45 which has a relatively low flow rate capacityas compared to the volume of chamber 22. This low flow rate capacityprovides good control at the low end of flow rate demand, and a boosterassembly 88 is also provided to enable increasing or boosting of theflow rate through valve 45 as flow rate demand increases, as will be setforth in detail below.

Thus, valve assembly 44 may include an inlet valve 45 having a valveactuator 47, such as a pneumatic or electrical actuator. A pneumaticactuator is shown. In addition, valve assembly 44 also preferablyincludes an exhaust valve 51 having a pneumatic actuator 53. Valves 45and 51 preferably operate in a “split range” configuration. That is tosay, inlet valve 45 and exhaust valve 51 are both closed at a certainintermediate pneumatic control pressure value (e.g., 9 PSI). Inlet valve45 progressively opens when its actuator 47 receives an increasinglyhigher pneumatic control signal (e.g., ranging from about 9 to about 15PSI), while exhaust valve 51 progressively opens when its actuator 53receives an increasingly lower pneumatic control signal (e.g., rangingfrom about 9 to about 3 PSI). Between zero and 3 PSI exhaust valve 51 isfully open and inlet valve 45 is fully closed. It is a feature of thepresent invention, therefore, that the split range configuration ofvalves 45 and 51 results in a fail-safe condition in which chamber 22 isnot pressurized in the absence of a pneumatic control signal.

Many prior art molten metal holding and pouring apparatus have attemptedto control pouring from the holding vessel by primarily sensing andusing the gas pressure in the metal containment chamber. The gases usedto pressurize the chamber, of course, have little mass and arecompressible, which makes the gas behave like a spring, making relianceon the chamber pressure alone unsatisfactory. The problem is exacerbatedby the fact that the molten metal has great mass and inertia, and isincompressible, so that the use of “springy” compressed gas to bringabout pouring can easily cause undesirable surging and oscillating,making attempts at automatic controls more difficult. When the gases inthe chamber are exhausted to atmosphere and a new pour cycle is quicklyattempted, gas compression can cause metal surging making the pourcontroller lose its ability to achieve accurate pours.

Prior art attempts to supplement the pour controller with a sensor whichsenses the level of the metal in the pour spout, usually by means of anelectrical contact and withdraw sensor or a float, has only marginallyimproved the accuracy of the pours. Such prior art pour spout levelsensors, which are only used with low temperature molten metals likealuminum and zinc, cannot be used with higher temperature ferrous andcopper base molten metals. Finally, as previously described, the use ofindirect level sensing of molten metal in the pour spout has proven tobe insensitive, ineffective and inaccurate.

In one aspect of the molten metal pouring apparatus of the presentinvention, therefore, a controller assembly is employed in which directdistance sensing of the level of the molten metal in the chamber iscombined with pressure measurements to effect more accurate control ofmetal pours. In another aspect, still further improvement results withlevel sensing of the metal in the pour passageway. In still a furtheraspect, a pressure control assembly is employed in which a low flow ratecapacity inlet valve is combined with a flow rate booster assembly toachieve improved pressure control, resulting in reduced surging andoscillation at both the low and high ends of flow rate demand.

The control assembly of the present invention, therefore, includes asensitive, fast-acting pressure sensor or transducer 54 formed andpositioned to sense and monitor the gas pressure in conduit 40 and thusin chamber 22 above molten metal charge 14. For example, apressure-to-electric transducer manufactured by Transicoil ofNorristown, Pa. would suffice in this regard. Transducer 54 could alsobe located in chamber 22, and it preferably is formed to produce anelectrical signal which is communicated via line or conductor 56 to asumming junction 58.

One key to accurate performance of the control assembly of the presentinvention is that direct sensing of the level of molten metal isemployed, rather than indirect sensing through the use of a load cell orbubbler tube. Thus, the control assembly of the present inventionincludes a vessel chamber distance sensor or detector 50 which directlysenses the distance from the sensor to the top surface 39 of the moltenmetal in chamber 22. Sensor or transducer 50 thus effects a distancesensing measurement without contacting the corrosive and destructivemolten metal. Sensor 50 can include a pressure-tight observation tube 55which is mounted to vessel cover 16 and extends into chamber 22 ofvessel 12. Chamber distance sensor 50 may advantageously be awave-operated distance sensing device such as a radar based sensor,which will be discussed in more detail below. Sensor 50 determines thelevel of the top surface 39 of molten metal charge 14 in vessel chamber22 throughout the pouring cycle.

In addition, holding vessel 12 will normally include a metal receivinginlet 41 with filling passageway 24 positioned higher than pour spoutoutlet 43 of pour passageway 26 to allow recharging of chamber 22 whileit is pressurized. Chamber distance sensor 50 is formed to produce asignal which is communicated through conductor 57 to a summing junction58 and to a computational module 90. The chamber distance sensor signalis adjusted to decline in proportion to the dropping of the level ofmetal surface 39.

Thus, accurate metal level sensing in chamber 22 determined bynon-contact distance measurements can be combined with the chamberpressure measurements to produce enhanced control of the pour cycle. Theprecision of successive pours can be further enhanced by employing amolten metal level sensor at the top of pour passageway 26. The controlassembly of the present invention, therefore, also preferably includes apour spout distance sensor 46 formed and positioned to sense thedistance to the top surface of the metal in the pour spout. Pour spoutdistance sensor 46 preferably includes an observation tube 48 which ismounted to vessel 12 and extends into mouth 28 of pour passageway 26.Sensor 46 determines the level of molten metal in pour passageway 26 andgenerates a signal representing the same. The pour spout sensor signalis communicated down line or conductor 49 to a timer 76 or, alternately,to integration module 77 (shown in broken lines). In particular, sensor46 senses the level of molten metal in pour passageway 26 rangingbetween a “ready-to-pour” level and a maximum “pour” level, which willbe discussed hereinafter. It will be appreciated that a “contact andwithdraw” level sensor 46, as for example is shown in U.S. Pat. Nos.3,998,365 and 4,220,319, also can be used as a pour spout distancesensor, particularly when lower temperature molten metals, such asaluminum and zinc, are being poured.

Sensors 46 and 50 are most preferably distance measuring sensors whichcan sense the level of the top surface of the molten metal withouthaving to contact the same so as to be less susceptible to damage by thehigh temperature molten metal charge 14. Advantageously, sensors 46 and50 are wave-operated sensors which may employ radar, a laser beam orother optics, or sonic or ultrasonic waves, for example, in making leveldeterminations. A capacitance based sensor also could be employed.Radar-type sensors available under the trademark Sitrans LR,manufactured by Siemens, are particularly well suited for both sensors46 and 50. Radar sensors are less affected by smoke, dust, steam, slag,and high molten metal temperatures than many other remote sensingtransducers.

While a radar sensor is preferred for use at pour spout 26, in thebroadest aspect of the present invention electrical “contact andwithdraw” type sensors, as shown in the prior art, also can be employedto sense any of the discrete “ready-to-pour,” “threshold of pouring,” or“pour” levels that are used in combination with direct level sensing andpressure sensing in chamber 22 in controlling pouring from vessel 12.Chamber distance sensor 50, however, preferably should not employelectrical contact distance sensing because of the unsuitableenvironment for such sensors, and since there are an infinite number oflevels in chamber 22 which need to be sensed for maximum accuracy.

In order to achieve a process variable control signal that is equivalentto the level of molten metal in pour passageway 26, the control assemblyof the present invention includes an electrical summing junction 58 forcombining the chamber distance level sensor signal with the chamberpressure sensor signal. Accordingly, electrical summing junction 58receives an electrical signal via conductor 57 from vessel chamberdistance sensor 50, as well as the signal from pressure transducer 54,representative of the gas pressure in conduit 40 and thus in chamber 22,through conductor 56. The output of summing junction 58 is the processvariable signal that is communicated through conductor 60 to controller80. This process variable signal is the electrical equivalent of theheight of molten metal in pour passageway 26. That is to say, when thepressure transducer signal equals zero, the process variable signalsolely equals the level 39 of molten metal 14 within chamber 22, assensed by the chamber distance sensor 50, which will be the same levelas the molten metal in pour spout 26. However, as the pressure of gaswithin chamber 22 of vessel 12, as monitored by pressure transducer 54,is increased, the height of molten metal 14 in pour passageway 26increases directly in proportion to the sensed gas pressure, while theheight of molten metal in the chamber decreases. Such changing in levelof molten metal 14 in pour passageway 26 and reduction in the height inchamber 22 will be reflected in the process variable signal from summingjunction 58.

A set point module 62 may be used to produce an adjustable “set point”output signal in conductor 78, which set point signal is electricallyproportional to the desired height of molten metal in pour passageway26. Set point module 62 includes set point 64 representing a“ready-to-pour” level 66 as shown in broken lines in FIG. 2. Inaddition, a “pour level” set point 68 is provided in set point module 62and corresponds to a “pour level” 70 of metal 14, illustrated in brokenlines in FIG. 2. Either set point 64 or set point 68 is electricallyselected by relay means 72 within set point module 62. The quantity ofmetal egressing from mouth 28 of pour passageway 26, per unit time, maybe adjusted by varying set point 68, which changes pour level 70 ofmolten metal in pour passageway 26. In addition, set point 68 also maybe varied to change the rate of pouring as molten metal egresses frommouth 28 of the pour passageway 26.

In order to enhance pouring control accuracy, the signal from pour spoutdistance sensor 46 may be communicated through conductor 49 to a timer76. When an external signal to begin pouring is received by thecontrols, relay 72 in set point module 62 switches from the “ready topour” set point 64 to the “pour level” set point 68. As pouringcommences, the pour spout level signal 49 from transducer 46 provideselectrical confirmation that molten metal in pour passageway 26 hasreached the “threshold of pouring” level, namely level 74 shown inbroken lines in FIG. 2, also known as the “drip point.” Timer 76 is usedto determine the quantity of molten metal poured. In this regard, timer76 receives the level signal from pour spout distance sensor 46,indicating that molten metal has reached the “drip point.” or “thresholdof pouring” level 74, which initiates the pouring time interval of timer76. Upon the expiration of its preset time interval, timer 76 signalsinternal relay 72 of set point module 62 to return to “ready-to-pour”level 66 as determined by set point 64.

Alternatively, pour spout distance sensor 46 may send a level signal toan integration module 77, shown in broken lines in FIG. 1, instead of totimer 76. The continuous output signal from level sensor 46 would thenbe used by integration module 77 to produce an accurate quantitativemeasurement of the amount of molten metal poured, beginning the momentthat molten metal in pour passageway 26 exceeds the “threshold”elevation 74. FIG. 1 depicts such alternate arrangement in broken lineswherein integration module 77 connects to set point module 62. Whenintegration module 77 reaches its preset limit, thereby indicating thatthe desired amount of molten metal has been poured from pour passageway26, integration module 77 signals relay 72 to switch from “pour” levelset point 68 back to “ready-to-pour” level set point 64.

The output signal from set point module 62 is communicated to controller80 through conductor 78, and the process variable signal from summingjunction 58 is also input to the controller through conductor 60.Controller 80 preferably is a suitable 3-mode (rate, reset andproportional band) controller which is capable of providing anelectrical output that varies with the rate of change, integral overtime, polarity and magnitude of the difference of the two input signalsreceived. Of course, controller 80 may produce other types of outputsignals, e.g., pneumatic signals, etc. In other words, controller 80compares the process variable signal from summing junction 58 to theselected set point signal from set point module 62. Any differencebetween the two signals is the “error” or differential signal, whichcauses controller 80 to produce an output signal in conductor 82 thattravels to a fast-acting electric to pneumatic converter 84 viaconductor 82.

Converter 84 directly controls valve assembly 44, specifically valves 45and 51 through actuators 47 and 53, respectively, as heretoforedescribed. As shown in the preferred embodiment of FIG. 1, converter 84is powered by compressed gas from source 42 via regulator 86. A separatesource of compressed gas also may be employed to operate regulator 86.Also, a motor operated pressure regulator with a position indicatingslide wire may be employed as converter 84 and valve assembly 44.Alternatively, valves 45 and 51 may be solenoid operated by anelectrical signal directly from controller 80. An electro-pneumaticconverter Model 870022, manufactured by DeZurik, can be employed as thefast-acting electric to pneumatic converter 84.

A reduction in pressure oscillations and surging in chamber 22 also isachieved in the present invention by selecting an inlet valve 45 thathas relatively low flow rate capacity, and then boosting the flow rateof the valve, as metal level 39 diminishes, with a separate boosterassembly. This combination widens the valve's operating range to includethe entire volume of vessel chamber 22 when it is nearly empty,effectively circumventing the valve's finite turndown ratio. Therelatively low flow rate capacity inlet valve permits controller 80 tomake minute corrections without directing too much or too little gasinto chamber 22, thus reducing or eliminating surging and oscillationsthat would otherwise, as in the prior art, be caused by applying thesesame corrections to a necessarily larger flow capacity valve notequipped with a separate booster assembly. During the time intervalsbetween successive pours, controller 80 directs converter 84 to maintaina typical pressure control signal to valves 45 and 51 between 9.5 and10.5 PSI. Because of the split range configuration of valves 45 and 51,this means that exhaust valve 51 is completely closed and inlet valve 45is slightly open, which maintains adequate pressure in chamber 22 tosteadily keep the level of metal in pour passageway 26 at the“ready-to-pour” level 66. When there are leaks from chamber 22,controller 80 will automatically increase the pressure control signal bywhatever amount is needed to simultaneously maintain the “ready-to-pour”level 66 and overcome the leaks.

Such stable control can best be achieved by using an inlet valve 45which has a low flow rate capacity relative to the volume of chamber 22.For example, in one embodiment a chamber 22 having a volume of 36 cubicfeet was controlled by an inlet valve 45 having a flow coefficient, orC_(V), of 3.7. Such a low flow coefficient, however, becomes a limitingand disadvantageous feature as the demand for flow increases, as isdescribed below. However, if inlet valve 45 is made larger in order tosatisfy the high flow rate required when metal level 39 in vesselchamber 22 is low, making the gas volume above the metal correspondinglylarge, pressure oscillations due to control feedback are experienced atthe low end of flow rate demand, where the system needs to operate mostof the time, because of the larger valve's inability to preciselymodulate lower levels of gas flow when directed by controller 80.

It has been found that the flow coefficient or C_(V) of inlet valve 45will substantially scale relative to the chamber 22 volume according toBoyle's Law, namely,(C _(V1) ×T ₁)/V ₁=(C _(V2) ×T ₂)/V ₂

Thus for a constant temperature, the above example can be scaled up ordown with changes in the volume of chamber 22 in order to select a“relatively small” or “low flow rate capacity” inlet valve 45.

The above example, however, is not necessarily regarded as beingabsolutely optimal, and one skilled in the art will recognize that bothlarger and smaller ratios of C_(V) to V can be employed. But as theratio of C_(V) to V increases, at some point the CV will be large enoughto begin to introduce pressure oscillations in chamber 22 that willinterfere with, or degrade, the pressure control. As used herein,therefore, the expression “relatively low flow rate capacity” shall meanan inlet valve having a flow rate capacity which is sufficient tominimize or substantially eliminate pressure oscillations in chamber 22due to control feedback in operation of valve 45.

While use of a relatively low flow rate capacity inlet valve allows thelow end of flow control to be enhanced, such an inlet valve by itselfcannot adequately respond to system high end flow demand. One problemwhich has existed in prior molten metal pouring apparatus is that, asthe level of molten metal drops in the vessel chamber, it requires moretime for a larger quantity of gas to flow from the source of pressurizedgas, in order to reach the pressure in the chamber necessary to producethe desired pour level in the pour spout. Low flow rate inlet valve 45,like all such valves, has a limited range of operation, commonly knownas its “turndown ratio.” Without a booster mechanism to effectivelyextend its range, this “turndown ratio” will restrict the ability ofvalve 45 to admit increasingly larger quantities of compressed gas tochamber 22, in the same amount of time during each pour, as the level ofmetal 39 diminishes due to pouring. The pressure source might, forexample, provide compressed gas at 75 to 150 PSI, but a fixed regulatoror regulators would normally be employed in the prior art to reduce thepressure to say 8 to 10 PSI to avoid molten metal surging and pourovershoots. Then, if the level of molten metal in chamber 22 were todrop sufficiently low, controller 80 would open valve 45 until it waswide open, delivering its maximum flow rate to the chamber. However,because valve 45 is selected to be a low flow rate valve for stablelow-end flow rate control, this flow rate is highly likely to beinsufficient to provide adequate gas flow to chamber 22 for low metallevels because of the limitation imposed by the turndown ratio of valve45. For this reason the apparatus of the present invention preferablyincludes a pressure booster or variable regulator assembly to overcomethis problem.

Booster mechanism 88 may include a computation module 90 which receivesthe chamber level signal from sensor 50 via conductor 57. Computationmodule 90 produces an electrical first order linear output signal whichis communicated through conductor 92 to an electric-to-pneumaticconverter 94. The linear output signal is inversely proportional to theinput from chamber distance sensor 50 according to the equation:y=mx+bwhere “y” is the output of computation module 90, “x” is the input fromchamber level sensor 50, “m” is the adjustable slope of a curve (whichwill be negative), and “b” is the adjustable Y-intercept whichdetermines the module 90 output when the input from sensor 50 is zero,corresponding to the lowest acceptable level of metal in chamber 22 ofvessel 12.

The electrical output signal from computation module 90 is sent toelectric-to-pneumatic converter 94, similar in structure to converter84. Converter 94 may share pneumatic power with converter 84 viaregulator 86. A gas volume booster relay 96 produces a high volume flowrate at a pressure equal to a reference pressure input signalcommunicated from converter 94 by conduit 98. When signaled byelectric-to-pneumatic converter 94, volume booster relay 96 increasesthe pressure at the input side of inlet valve 45, which permitscompressed gas from source 42 to flow through control valve 45 at a veryhigh rate, for example, well above the rate limited by the turndownratio of valve 45 at the unboosted pressure which would otherwise besupplied to the valve. A volume booster relay Model 4500, manufacturedby Fairchild can be used as volume booster relay 96. Pneumatic converter94 would take the same commercial form as converter 84.

Booster mechanism 88, therefore, greatly extends the useful range ofoperation of valve 45 and reduces the time required for pour cycles whenthe metal level in chamber 22 drops to lower levels by increasing thegas pressure available to valve 45. The increase from booster 88,however, is gradual and does not increase the pressure available tovalve 45 to a level causing pressure oscillations at the low end of flowrate demand. The booster, however does permit valve 45 to admit therequired volume of compressed gas into chamber 22 in the same amount oftime during each pour at both the high and low ends of flow rate demand,with a corresponding improvement in overall system pouring accuracy andefficiency. As previously noted, the function and operating range ofbooster mechanism 88 is determined by the settings “m” and “b” incomputational module 90.

In operation, holding vessel 12 receives a charge 14 of molten metal ofany desirable genre via charging inlet 24. To minimize surging of moltenmetal during successive pours, and the disadvantages accruing thereto,it is desirable to initially hold the molten metal in vessel 12 under apressure in chamber 22 causing the metal in pour passageway 26 to beraised to “ready-to-pour” level 66 in FIG. 2 prior to pouring.Accordingly, when pouring apparatus 10 is initiated, an “error” signalis immediately detected at the input of electronic controller 80, basedon the fact that the process variable signal is less than“ready-to-pour” set point 64. A rising output signal from controller 80is received by electric-to-pneumatic converter 84. This produces arising pneumatic signal from converter 84 which, in turn, causes exhaustvalve 51 to close and gas inlet valve 45 to open. This results in anincrease in gas pressure in chamber 22 of vessel 12, and a correspondingrise in the level of molten metal 14 in pour passageway 26. The gaspressure within chamber 22 will continue to increase until the processvariable signal 60 entering controller 80 exactly equals “ready-to-pour”set point signal 78 from set point module 62, and molten metal 14reaches “ready-to-pour” level 66 in passageway 26. When this occurs theoutput of controller 80 will stop the increase of pressure withinchamber 22 of vessel 12 and molten metal 14 will stay at “ready-to-pour”level 66 in pouring tube 26.

The command to actually pour molten metal 14 from pour passageway 26,directional arrow 100 in FIG. 1, would commence upon the receipt of anexternal signal from an adjacent molding or casting machine or othercasting equipment (not shown). Such command causes the set point relay72 within set point module 62 to switch from “ready-to-pour” set point64 to “pour” set point 68. The resulting new “error” signal is detectedby controller 80 which generates, via converter 84 and valve assembly44, an increase in gas pressure through conduit 40 to chamber 22 ofvessel 12. When the molten metal in the pour passageway reaches thethreshold level 74, the presence of molten metal at this level is sensedby pour passageway level sensor 46, which then starts electronic timer76, and pouring actually begins. Pouring continues for a user-presetinterval of time and at a user-preset pouring rate determined by the“pour” set point 68 which controls pour level 70. FIG. 3 schematicallydepicts the control process above described.

It should be noted that pour passageway distance level sensor 46eliminates a major component of dynamic pouring error by starting thepour interval timer 76 when the molten metal in passageway 26 actuallyreaches “threshold of pouring” level 74 instead of beforehand, i.e.,instead of when a signal to begin pouring is first received, as in theprior art. This feature greatly reduces dynamic system errors derivedfrom variations in the time required for molten metal to move from“ready-to-pour” level 66 to “threshold of pouring” level 74.

FIG. 3 illustrates the advantage of holding the metal at the“ready-to-pour” level and not starting the timer or integration untilthreshold of pouring level 74 is actually reached. Abscissa 102 in FIG.3 represents time, generally measured in seconds, and ordinate 104represents the height of molten metal 14 in pour passageway 26 asdetected by pour passageway distance sensor 46. At origin 106, to isdefined as the instant that action to initiate the pouring cycle begins,and h₀ is the “ready-to-pour” level 66 in pour passageway 26. The heighth₁ is the top of weir 108 of pour passageway 26 when the metal is atthreshold of pour level 74 as shown in FIG. 2, and h₂ represents theuser input or selected “pour” level 70. It should be realized that theflow of any liquid including molten metal over weir 108 or,alternatively, through an orifice (not shown) is directly proportionalto the height of molten metal above the weir 108 or orifice, as afunction of time.

The dynamic time intervals during which the metal level in pourpassageway 26 moves from level 66 to level 70 and back again are definedby the time intervals t₀ to t₂ and t₃ to t₅ respectively. These dynamictime intervals are more difficult to control, on a repeatable basis,than the steady state time intervals such as those between t₂ and t₃,after t₅, and prior to t₀. This means that the amount of metal 14leaving mouth 28 of pour passageway 26 during the dynamic control phaseswill contain a larger component of error per unit time than the amountof metal 14 poured during steady state control phases. However, pourpassageway distance sensor 46 is able to detect level 74, the thresholdlevel, enabling timing device 76 to start at time t₁ rather than at t₀,as found in the prior art. In this way, all dynamic control errorsbetween h₀ and h₁ are eliminated. Since this is the largest errorcomponent of the dynamic control phase, overall pouring errors aresignificantly reduced.

As above noted, timer 76 may be replaced by electronic integrationmodule 77 that operates in conjunction with a continuous output signaloriginating from pour passageway level sensor 46. Again, an adjacentmolding machine or casting machinery (not shown) would initiate pouringby causing relay 72 to switch from “ready-to-pour” set point 64 to the“pour” set point 68. When the level of metal in pour passageway 26increases above the “drip point” or “threshold of pouring” level 74, (h₁of FIG. 3), integration module 77 begins calculating the amount ofmolten metal actually dispensed or poured “on the fly” by integratingthe molten metal level h (FIG. 3) over time as continuously reported byoutput signals from level sensor 46. When integration module 77determines that the quantity of molten metal, preset by the user, hasbeen dispensed, it signals the electric relay 72 to switch back to the“ready-to-pour” set point 64. Thus, the smaller steady state controlsystem errors that affect the actual pour level h₂ when only a timer isemployed are also reduced by use of integration, further increasingoverall system accuracy. When a desired quantity of metal to bedispensed is preset in integration module 77, an allowance must be madefor the small but repeatable quantity of metal represented by area 110on FIG. 3, which is poured when the metal in pour passageway 26 dropsfrom h₂ back to h₁. The integration module precisely terminates the pourshort of the desired volume by this amount, as represented by area 110.

Controlling the quantity of metal poured through this integrationtechnique is particularly useful when pour level h₂ is purposely changedduring the pour itself to accommodate individual mold pouringrequirements.

Because of the small height differential between “ready-to-pour” level66 and “pour” level 70, FIG. 2, the partial pressurization of chamber 22between successive pours, and the behavior of closed loop, servocontroller 80 when properly adjusted, the transition between these twolevels is rapid and very smooth. Thus, the quantity of compressed gasneeded to complete each pour together with the pouring cycle time areconsiderably reduced, and pressure oscillations and molten metal surgingare also minimized.

It should be noted that when the system returns to “ready-to-pour” setpoint 64 in set point module 62, it begins to operate in reverse sincean “error” signal of opposite polarity is received by controller 80.Controller 80 then decreases its output signal 82 to converter 84, whichin turn causes exhaust valve 51 to dump only a small portion of thecompressed gas contained in chamber 22 of vessel 12 via conduit 40. Thisaction takes place until metal 14 in pouring tube 26 returns to“ready-to-pour” level 66.

When molten metal is charged into chamber 22 of holding vessel 12 viainlet passageway 24, the higher level of molten metal 39 in chamber 22is detected by vessel chamber distance sensor 50. The process variablesignal then increases resulting in a decrease in the gas pressuresupplied to chamber 22 of vessel 12. In other words, the processvariable signal 60 remains equal to the desired height of molten metalin pouring tube 26 and, via controller 80, equal to the set pointselected in set point module 62, even when the vessel is being refilled.

It will be apparent, therefore, that the pressure control systemdescribed herein is a true closed loop, servo-type control system whichwill act to self-correct for a number of system variables, includingnaturally occurring gas leaks from the vessel chamber, whichsignificantly contributes to its reliability and accuracy.

The present invention makes it possible to pour very accurate quantitiesof molten metal in a wide range of flow rates because the flow rate ofmetal from the pour passageway 26 of vessel 12, as well as the timerequired to induce that flow rate, remains constant, regardless of thelevel of molten metal in the vessel.

While the preferred embodiment of the present invention is based upon achamber distance sensor 50 which directly senses the level of moltenmetal charge in chamber 22, it is also possible to attain many of theadvantages of the present invention by employing indirect sensing of thelevel of molten metal in chamber 22, particularly when the chamberconfiguration is uniform so that a substantially linear relationship ofweight-to-charge level exists. Thus, instead of vessel chamber distancesensor 50, the apparatus of the present invention can employ a load cell38 which automatically determines the weight of charge 14 by subtractingout the tare weight of vessel 12 and its components from the grossweight reading obtained. Load cell 38 then produces an electrical signalanalogous to the level of molten metal in the vessel chamber 22 which iscommunicated via conductor 52 to summing junction 58 and computationalmodule 90. It should be noted that load cell 38 and pressure transducer54 require accurate calibration and adjustment upon installation insystem 10 of the present invention with each particular application.After initial calibration, very little adjustment is required tomaintain the balance of apparatus 10 of the present invention. Use ofindirect or load cell sensing of the level of molten metal in chamber22, however, is not preferred and can be problematic for low densitymetals, as above noted. However, error can be minimized by combiningpour passageway level sensing with load cell based chamber levelsensing.

Turning now to FIG. 4, a block diagram of the preferred embodiment ofthe above-described molten metal holding and pouring apparatus 10 isshown. Vessel 12 has a chamber 22 for charge 14, and direct distancesensors 46 and 50 are provided to sense the molten metal levels in thepour passageway and the chamber, respectively. The sensed level in thechamber is combined with the pressure in the chamber sensed bytransducer 54 at summing junction 58 to produce a process variablesignal. Controller 80 compares the process variable signal to set pointsignals from set point module 62. The controller controls operation ofgas valve mechanism or assembly 44 to admit gas from source 42 intochamber 22. Level signals from sensor 46 determine the level of moltenmetal in the pour passageway and start a timing or integration process(76/77) which, at its conclusion, signals the set point module 62 tocause controller 80 to exhaust gas from vessel chamber 22 at the end ofthe pour cycle. Finally, computational module 90 causes booster 96 toincrease the flow of compressed gas through valve assembly 44 to vesselchamber 22 as the level of molten metal charge 14 diminishes, asdetermined by vessel chamber level sensor 50.

From the foregoing description of the present apparatus and itsoperation, it will be seen that the method of controlling pouring ofmolten metal from metal holding vessel 12 includes the steps of sensingthe distance to top surface 39 of metal charge 14 in chamber 22, sensingthe gas pressure in the chamber, and pressurizing the chamber over themolten metal in response to a combination of the sensed distance andpressure by an amount producing a controlled outflow or pour of moltenmetal from chamber 22 through pour passageway 26. The distance sensingstep is preferably accomplished without contacting the metal charge inchamber 22 by employing a wave-operated distance sensor such as a radarsensor.

The present method also preferably includes a sensing step in whichlevel of the molten metal in the pour spout is also sensed and used toprecisely detect the beginning and control the duration of the pour.Again this is most preferably accomplished using a wave-operateddistance sensor 46 which does not contact the molten metal.

In another aspect of the present method, a step of selecting arelatively low flow rate capacity inlet valve is accomplished for lowend flow rate control and the step of boosting the rate of the admissionof gas into chamber 22 is undertaken as the level of molten metal 14 inchamber 22 diminishes for high end flow rate control. This pressureboosting step allows constant pour cycle times to be maintained as thecharge in the holding vessel decreases.

It should be noted that, in the interest of simplicity and clarity, theabove system has been described as being comprised of individual,discrete components and modules. However, this description is notintended to be limiting in this regard since many of these componentsand modules can be readily obtained as part of commonly used industrialprogrammable logic controllers (PLCs) or industrial grade computers,which are highly recommended for this and similar applications.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since numerous changes may be made in the above construction withoutdeparting from the spirit, scope and principles of the invention, it isintended that all matter contained in the above description and shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

1. A molten metal holding and pouring apparatus comprising: a holdingvessel having a sealable molten metal containing chamber with a pourpassageway extending from the chamber to a pour spout outlet; a gassupply assembly formed and coupled to pressurize the chamber in order tocontrol the level of molten metal in the pour passageway; and a controlassembly including a pressure sensor formed and positioned to sense thepressure in the chamber above the molten metal, a chamber distancesensor formed to sense the distance to the top of the molten metal inthe chamber, and a controller coupled to the pressure sensor and thechamber distance sensor to receive sensor signals therefrom and coupledto the gas supply assembly, and the controller being responsive tosignals received from the sensors to cause the gas supply assembly topressurize the chamber to control the level of molten metal in the pourpassageway and to control the outflow of molten metal from the vessel.2. The apparatus as defined in claim 1 wherein, the control assemblyfurther includes a pour spout level sensor formed and positioned tosense the level of molten metal in the pour passageway.
 3. The apparatusas defined in claim 2 wherein, the pour spout level sensor is a distancesensor formed and positioned to sense the distance to the top surface ofthe molten metal in the pour passageway.
 4. The apparatus as defined inclaim 2 wherein, the pour spout level sensor is a contact and withdrawlevel sensor.
 5. The apparatus as defined in claim 1 wherein, the gassupply assembly includes a relatively low flow rate capacity inlet valvefluid coupled to control of the flow of pressurized gas to the chamber,and the gas supply assembly includes a gas supply booster device formedto boost the pressure of the gas at the inlet valve to effect anincrease in the flow rate of gas through the inlet valve as the sensedlevel of molten metal in the chamber diminishes.
 6. The apparatus asdefined in claim 5 wherein, the booster device increases the flow rateof gas through the inlet valve continuously and in an increasing manneras the level of molten metal in the chamber lowers.
 7. The apparatus asdefined in claim 1 wherein, the chamber distance sensor is provided by awave-operated distance sensing device.
 8. The apparatus as defined inclaim 7 wherein, the wave-operated distance sensing device employs radarfrequency waves to sense distance.
 9. The apparatus as defined in claim3 wherein, the chamber distance sensor and the pour spout distancesensor are both wave-operated distance sensing devices.
 10. Theapparatus as defined in claim 1 wherein, the gas supply assemblyincludes a source of pressurized gas, a conduit array coupling thesource of compressed gas to the chamber, a valve assembly mounted in theconduit array and formed to control the gas pressure supplied to thechamber, and wherein the pressure sensor is positioned to sense gaspressure in at least one of the conduit array and the chamber.
 11. Theapparatus as defined in claim 8 wherein, the pressure sensor is mountedin the conduit array.
 12. The apparatus as defined in claim 11, and abooster device mounted in the conduit array in advance of the inletvalve and formed to increase the pressure at the inlet valve to increasethe flow rate of gas to the chamber for any given inlet valve opening.13. The apparatus as defined in claim 12 wherein, the booster deviceincludes a computation module receiving control signals from the chamberdistance sensor representing the level of molten metal in the chamber,the computation module generating an output signal which is inverse tothe control signal received from the distance sensor, and the boosterdevice being responsive to the inverse output signal from thecomputational module to gradually increase the flow of pressurized gasfrom the source of compressed gas to the chamber as the level of moltenmetal in the chamber lowers.
 14. The apparatus as defined in claim 13wherein, the computation module output signal comprises an electricalsignal, and a converter coupled to receive the electrical signal andresponsive thereto to generate an analogous pneumatic computation modulesignal, the pneumatic computation module signal being fluid coupled toand operating the booster device.
 15. The apparatus as defined in claim10 wherein, the gas supply assembly further includes: a. a summingjunction formed to receive signals from the pressure sensor and thechamber distance sensor and formed to add the values of the signalsreceived to produce a process variable signal representative of thelevel of molten metal in the pour passageway; b. a set point moduleformed to and generating a set point signal representative of a desiredlevel of molten metal in the pour passageway; c. a controller receivingthe process variable signal from the summing junction and the set pointsignal from the set point module, and responsive thereto to generate acontroller signal as an output; and d. wherein the inlet valve isresponsive to the controller signal to control the flow rate of gas tothe chamber.
 16. The apparatus as defined in claim 15, and a timercoupled to maintain the set point signal for a predetermined duration oftime.
 17. The apparatus as defined in claim 15, and an integrationmodule coupled to maintain the set point signal until a predeterminedamount of molten metal has been poured from the vessel.
 18. Theapparatus as defined in claim 15, and a device formed and coupled tovarying the value of the set point signal during the passing of moltenmetal through the pour passageway.
 19. The apparatus as defined in claim15 wherein, the controller signal comprises an electrical signal, andwherein the inlet valve assembly further includes a converter forreceiving the electrical signal from the controller and converting theelectrical signal into a pneumatic output signal.
 20. The apparatus asdefined in claim 15 wherein, the inlet valve is located in the conduitarray entering the vessel for controlling the flow of pressurized gasinto the chamber, and an exhaust valve in the conduit array forexhausting pressurized gas from the conduit array.
 21. The apparatus asdefined in claim 20 wherein, the inlet valve and exhaust valve operateas a split range pneumatic control device responsive to the pneumaticoutput signal.
 22. The apparatus as defined in claim 21 wherein, thesplit range control device is formed to open the exhaust valve and closethe inlet valve in the absence of a control signal.
 23. The apparatus asdefined in claim 22 wherein, the split range control device closes theexhaust valve as the output signal increases from a low value to amid-range value and opens the input valve as the output signal increasesfrom the mid-range value to a maximum value.
 24. The apparatus asdefined in claim 1 wherein, the vessel includes a fill passagewaycommunicating with the chamber, the fill passageway including areceiving inlet positioned at an elevation higher than the pour spoutoutlet of the pour passageway, such that the fill passageway facilitatesthe refilling of the chamber with molten metal while the chambercontains pressurized gas.
 25. A molten metal holding and pouringapparatus comprising: a holding vessel having a sealable molten metalcontaining chamber with a pour passageway extending from the chamber toa pour outlet; a gas supply assembly formed and coupled to pressurizethe chamber in order to control the level of molten metal in the pourpassageway; and a control assembly including a pressure sensor formedand positioned to sense the pressure in the chamber above the moltenmetal, a chamber level sensor formed and positioned to sense the levelof molten metal in the chamber, a pour spout distance level sensorformed to sense the level of molten metal in the pour passageway, and acontroller coupled to the sensors to receive signals therefrom andcoupled to send control signals to the gas supply assembly, and thecontroller being responsive to the sensor signals to send controlsignals to the gas supply assembly pressurizing the chamber to controlthe level of molten metal in the pour passageway and to control theoutflow of molten metal from the vessel.
 26. The apparatus as defined inclaim 25 wherein, the pour spout distance level sensor is awave-operated distance sensing device.
 27. The apparatus as defined inclaim 26 wherein, the wave-operated distance sensing device utilizesradar frequency waves.
 28. The apparatus as defined in claim 25 wherein,the chamber level sensor is a distance level sensor.
 29. The apparatusas defined in claim 28 wherein, the chamber level sensor is awave-operated distance sensing device.
 30. The apparatus as defined inclaim 29 wherein, the wave-operated distance sensing device utilizesradar frequency waves.
 31. The apparatus as defined in claim 25 wherein,the gas supply assembly includes a computation module receiving a signalfrom the chamber level sensor representing the level of molten metal inthe chamber, the computation module generating an output signal which isinverse to the signal from the chamber level sensor, and a pneumaticbooster coupled to receive the computation module output signal, thepneumatic booster producing an increased flow of pressurized gas to thechamber as the level of metal in the chamber diminishes.
 32. Theapparatus as defined in claim 31 wherein, the computation module outputsignal is an electrical signal, and a converter coupled to receive theelectrical signal and formed for transforming the electrical signal intoa pneumatic signal, the pneumatic signal being fluid-coupled to operatethe pneumatic booster.
 33. The apparatus as defined in claim 31 wherein,the chamber level sensor is a weight sensing device.
 34. The apparatusas defined in claim 25 wherein, the control assembly further includes:a. a summing junction coupled and formed to add the values of an inputsignal from the pressure sensor and an input signal from the chamberlevel sensor, the summing junction being responsive to the input signalsto output a process variable signal representative of the level ofmolten metal in the pour passageway; b. a set point module generating asignal representative of a desired level of molten metal in the pourpassageway; c. a controller receiving the process variable signal fromthe summing junction, and receiving the set point signal from the setpoint module, and generating a control signal as an output; and d. avalve assembly being responsive to the control output signal from saidcontroller to control the pressurization of the chamber.
 35. Theapparatus as defined in claim 34, and a timer coupled to maintain theset point module signal for a predetermined duration of time.
 36. Theapparatus as defined in claim 34, and an integration module coupled tomaintain the set point module signal until a predetermined amount ofmolten metal has been poured from the chamber.
 37. The apparatus asdefined in claim 34 wherein, the set point module is formed forvariation of the set point module signal during the outflow of moltenmetal through the pour passageway.
 38. The apparatus as defined in claim34 wherein, the controller output signal is an electrical signal, andwherein the valve assembly further includes a converter for receivingthe electrical signal from the controller and converting the electricalsignal into a pneumatic signal.
 39. The apparatus as defined in claim 34wherein, the valve assembly includes an inlet valve located in theconduit array for controlling the flow of pressurized gas into thechamber and an exhaust valve located in the conduit array for exhaustingpressurized gas from the conduit array.
 40. The apparatus as defined inclaim 39 wherein, the inlet valve and the exhaust valve are fluidcoupled in a split range control configuration acting oppositely withrespect to one another in response to the pneumatic signal.
 41. Theapparatus as defined in claim 25 wherein, the pour spout distance levelsensor is an electrical contact and withdraw level sensor.
 42. Theapparatus as defined in claim 25 wherein, the chamber level sensor is aload cell mounted to sense the weight of the vessel and charge.
 43. Amolten metal holding and pouring apparatus comprising: a holding vesselhaving a sealable molten metal containing chamber with a pour passagewayextending from the chamber to a pour spout outlet; a gas supply assemblyformed and coupled to pressurize the chamber with a gas in order tocontrol the outflow of molten through the pour passageway; and a gassupply booster device fluid coupled to the gas supply assembly andformed to boost the pressure of the gas delivered to the chamber as thevolume of molten metal in the chamber decreases.
 44. The apparatus asdefined in claim 43 wherein, the gas supply assembly includes an inletvalve having a relatively low flow rate capacity, mounted between thebooster device and the chamber.
 45. The apparatus as defined in claim44, and a chamber level sensor mounted to sense the level of moltenmetal in the chamber and formed to produce a chamber level sensorsignal, the gas supply booster device being coupled to receive thechamber level sensor signal and being responsive thereto to boost theflow rate of pressurized gas delivered through the inlet valve to thechamber.
 46. The apparatus as defined in claim 45 wherein, the chamberlevel sensor is formed to sense the distance from the level sensor tothe top surface of the molten metal.
 47. The apparatus as defined inclaim 45 wherein, the chamber level sensor is provided by a weightsensing assembly.
 48. The apparatus as defined in claim 45 wherein, thebooster device includes a computation module receiving the chamber levelsensor signal representing the level of molten metal in the chamber, thecomputation module generating an output signal which is inverse to thechamber level sensor signal, and the booster device receiving thecomputation module output signal and being responsive thereto toincrease the flow of pressurized gas to the chamber as the level ofmetal in the chamber diminishes.
 49. The apparatus as defined in claim48 wherein, the computational module output signal is an electricalsignal, and a converter for transforming the electrical computationmodule output signal into a pneumatic computation module signal foroperation of the booster device.
 50. A molten metal holding and pouringapparatus comprising: a holding vessel having a sealable molten metalcontaining chamber with a pour passageway extending from the chamber toa pour spout outlet; a gas supply assembly formed and coupled topressurize the chamber with a gas in order to control the outflow ofmolten through the pour passageway; and a pour spout distance levelsensor formed and positioned to sense the level of the top surface ofmolten metal in the pour passageway and coupled to the gas supplyassembly.
 51. The apparatus as defined in claim 50 wherein, the pourspout distance level sensor is a wave-operated distance sensing device.52. The apparatus as defined in claim 41 wherein, the wave-operateddistance sensing device utilizes radar frequency waves.
 53. Apressure-based method of pouring molten metal from a metal holdingvessel comprising the steps of: sensing the distance from a sensor tothe top of a charge of molten metal in a sealed chamber of a moltenmetal holding vessel; sensing the pressure of a gas in the chamber overthe molten metal; and pressurizing the chamber over the molten metal inresponse to a combination of the sensed distance and the sensed pressureto produce a controlled outflow of molten metal from the chamber througha pour passageway.
 54. The method as defined in claim 53 wherein, thestep of sensing the distance is accomplished by employing awave-operated distance sensor.
 55. The method as defined in claim 53wherein, the step of sensing the distance is accomplished by employing aradar distance sensor.
 56. The method as defined in claim 53, and thesteps of: sensing the level of molten metal in the pour passageway; andemploying the sensed level of molten metal in the pour passageway tocontrol the duration of outflow of molten metal through the pourpassageway.
 57. The method as defined in claim 56 wherein, the step ofsensing the level of molten metal in the pour passageway is accomplishedusing a distance measuring sensor that does not contact the moltenmetal.
 58. The method as defined in claim 57 wherein, the step ofsensing the level of molten metal in the pour passageway is accomplishedusing a radar-based distance sensor.
 59. The method as defined in claim56 wherein, the step of sensing the level of molten metal in the pourpassageway is accomplished using a contact and withdraw level sensorassembly.
 60. The method as defined in claim 53 wherein, the sensingsteps and the pressurizing step provide a self-correcting closed loopcontrol process.
 61. The method as defined in claim 53, and the step of:repeating the pressurizing step to produce a plurality of successivecontrolled outflows of molten metal.
 62. The method as defined in claim61, and the step of: between each outflow of molten metal, maintaining apartial pressure over the metal in the chamber.
 63. The method asdefined in claim 53, and the step of: boosting the flow of compressedgas to the chamber as the level of molten metal in the chamberdecreases.
 64. The method as defined in claim 53 wherein, thepressurizing step is accomplished by controlling the flow of gas from asource of compressed gas to the chamber with a self-correcting, closedloop control circuit.
 65. The method as defined in claim 63 wherein, thepressurizing step is accomplished by controlling the flow rate of gasinto the chamber using an inlet valve having a relatively low flow ratecapacity.
 66. A method of pouring molten metal from a containment vesselhaving a sealable chamber and a fluid connected pour passagewayextending from the chamber comprising the steps of: pressurizing thechamber with a gas in response to a combination of a level of moltenmetal sensed in the chamber and the pressure sensed in the chamber tocontrol outflow of molten metal from the chamber through the pourpassageway; and sensing the level of molten metal in the pour passagewayto control the length of time of the pour.
 67. The method as defined inclaim 66 wherein, the step of sensing the level of metal in the pourpassageway is accomplished by employing a distance sensor withoutcontacting the molten metal.
 68. The method as defined in claim 67wherein, the step of sensing the level of metal in the pour passagewayis accomplished by employing a wave-operated distance sensor.
 69. Themethod as defined in claim 68 wherein, the step of sensing the level ofmetal in the pour passageway is accomplished using a radar sensor. 70.The method as defined in claim 66 wherein, the pressurizing step isaccomplished in response to a level of molten metal sensed directlyusing a distance sensor sensing the level from a position out of contactwith the molten metal.
 71. The method as defined in claim 70, and thestep of: mounting a relatively low flow rate capacity inlet valve tocontrol gas flow into the chamber, and boosting the rate of gas flowthrough the inlet valve to the chamber in response to a senseddiminished level of molten metal in the chamber.
 72. A pressure-basedmethod of pouring molten metal from a sealed chamber of a holding vesselcomprising the steps of: pressurizing the chamber to produce an outflowof molten metal from the chamber through a pour passageway; sensing thelevel of molten metal in the chamber; and boosting the rate ofpressurization of the chamber during the pressurizing step as the levelof molten metals in the chamber diminishes.
 73. A method of pouringmolten metal from a sealed containment chamber of a vessel through apour passageway comprising the steps of: a. pressurizing the chamber toeffect the steps of 1) first bring the level of molten metal in thepassageway up to a ready-to-pour level closely proximate and below anoutlet of the passageway; 2) thereafter raise the level of molten metalfrom the ready-to-pour level through the threshold of pouring level to apour level above the outlet for the outflow of molten metal from theoutlet; and 3) thereafter lower the level of molten metal in thepassageway to below the outlet level; and b. during the pressurizingstep, timing the interval from the moment the threshold of pouring levelis reached to the start of the lowering step to enable an accuratedetermination of the quantity of molten metal outflowed from thepassageway outlet.
 74. A method of pouring molten metal from acontainment vessel comprising the steps of: pressurizing a sealedchamber in the vessel containing a molten metal charge to urge moltenmetal out of the chamber and up a pour passageway to a ready-to-pourlevel in the passageway closely proximate an outlet of the passageway;increasing the pressure in the chamber to urge the level of molten metalup to a known pour level above the outlet of the passageway outlet; andcontinuing to increase the pressure in the chamber to maintain the levelof molten metal at the pour level for a period of time producing anoutflow of a desired quantity of molten metal from the outlet; andreducing the pressure in the chamber to allow the molten metal in thepassageway to fall below the outlet level.